Meshes more and more frequently provide the support for representing objects, particularly in a virtual reality scene. They thus represent most information describing such a scene. Conventionally, a mesh is defined by a set of vertices and faces (or a set of vertices, edges and orientations) defining a topology.
A large number of image compression techniques are known to reduce the quantity of data necessary to represent an image or a sequence of animated images. The objective is particularly to reduce digital signal rates, in order to transmit the signals and/or to store them on a data support.
At the same time, the image or the sequence of images displayed on a destination terminal must be of the best possible quality, or at least of a predetermined quality level.
It is therefore particularly important to improve the perceptual quality of images produced from meshes while minimizing the quantity of information produced.
The invention is particularly, but not exclusively, applicable to the display of images on a terminal, or the display of objects making up a three-dimensional scene during a progressive transmission.
The invention is equally applicable to any type of polygonal mesh connected to image or video type data that can be triangulated.
Subdivision surfaces are more and more frequently used in the field of graphics data processing due to their excellent efficiency in improving the rendering of curved surfaces, coding, transmission and modelling of images.
The techniques for subdivision of surfaces defined by triangular meshes are described particularly in the SIGGRAPH 99 document (“Subdivision for Modelling and Animation” by Denis Zorin). Triangular meshes can then be refined by interpolation, which keeps the vertices of the original mesh while generating a surface with a C1-differentiability (Dyn, Levin and Gregory, “A butterfly subdivision scheme for surface interpolation with tension control”, ACM Transactions on Graphics 9, 2 (April 90), 160–169), or by approximation.
Loop (“Smooth Surface Subdivision based on Triangles”, University of Utah, Department of Mathematics, Master's Thesis, 1987) proposed a method for generating triangular meshes by global 1 to 4 subdivision, in 1987. The surface thus obtained then has a C2-differentiability on each of its vertices, except on extraordinary vertices in the original mesh that have a C1-differentiability (an ordinary vertex is defined as being a vertex with a valence equal to 6, and an extraordinary vertex of a triangular mesh is defined as being a vertex with a valence not equal to 6).
Subsequently, Hoppe (Hoppe et al., “Piecewise Smooth Surface Reconstruction”, SIGGRAPH 94 Conference Proceedings) developed an adaptive version of the method proposed by Loop, in 1994. Use of this technique can produce a surface that keeps and reconstructs geometric singularities such as sharp edges, corners and pointed tips of objects. The result is a piecewise smooth surface with continuity on curves forming the sharp edges, while providing differentiability on surface elements making up the mesh.
A disadvantage of these techniques according to prior art is that it is impossible to use them to adapt the image to the view point of a virtual observer. In particular, these techniques cannot use silhouettes (in this case silhouette means all edges of the mesh sharing two faces, one of which is oriented facing a virtual camera, and the other is in the opposite direction), the pyramid of vision and the orientation of the faces of objects facing a camera or the eye of an observer. The main result is “overcoding” of areas that are only slightly visible or are not visible at all, or are less relevant, to the detriment of visually important areas.
Another disadvantage of these techniques according to prior art is that they cannot enable different detail levels to cohabit on a triangular mesh representing an object.
Another disadvantage of these techniques according to prior art is that they make it impossible to optimise the ratio of the perceptual quality to the required information quantity (in other words, for example, the number of triangles necessary to display an image on a graphic terminal.).
Another disadvantage of most of these techniques is that they use a 1 to 4 subdivision operator. This type of operator has a multiplication factor of 4, which limits the harmonious cohabitation of different detail densities within the image, and generates coarse pixelisation. Furthermore, these techniques very quickly become expensive in memory if it is required to improve the pixelisation of the object.
In particular, the purpose of the invention is to overcome these disadvantages according to prior art.
More precisely, one purpose of the invention is to provide a process for a visually continuous refinement of triangular meshes.
Another purpose of the invention is to implement a process for refinement of triangular meshes depending on the view point of a virtual observer.
Another purpose of the invention is to provide a process for refinement of triangular meshes that optimises the ratio of the perceptual quality to the information quantity.
Another purpose of the invention is to implement a process for refinement of triangular meshes that is particularly suitable for curved surfaces.
Another purpose of the invention is to provide a process for refinement of triangular meshes adapted to surfaces comprising angular areas to be kept, and to preserve geometric singularities such as corners or sharp edges.
Another purpose of the invention is to implement a fast, robust and reversible process for refinement of triangular meshes.
Another purpose of the invention is to provide a process for refinement of triangular meshes that is simple and inexpensive to implement.
Another purpose of the invention is to implement a process for refinement of triangular meshes that can be adapted to the terminal on which the image or sequence of images is synthesized and then displayed.
Another purpose of the invention is to provide a process for refinement of triangular meshes that can give a refined grid.
Another purpose of the invention is to implement a process for refinement of triangular meshes to obtain harmonious cohabitation of different levels of detail within the displayed image.